Prospecting for off-shore hydrocarbon reservoirs relies heavily on seismic data acquisition, even more so than in the on-land case, due to the high cost of off-shore drilling compared to on-land drilling. It is therefore even more important in the field of marine seismic prospecting that the surveys be accurately and efficiently performed, so that high resolution surveys may be performed over reasonable time and at reasonable cost.
A common method for marine seismic surveying uses a vessel to tow multiple hydrophone streamers in the area of a survey. The towed array collects reflected (and, in some cases, refracted) signals from sub-surface strata, and which originated from a conventional seismic source such as an air gun. The seismic source may be towed behind the same vessel as the hydrophone streamers, towed behind a different vessel, or may be fixed in position by way of a buoy or anchor An example of a method for marine seismic prospecting using towed hydrophone arrays, particularly for performing three-dimensional (3-D) prospecting, is described in U.S. Pat. No. 4,970,696issued Nov. 13, 1990, assigned to Atlantic Richfield Company and incorporated herein by this reference; this patent particularly utilizes separate source boats to accomplish the 3-D survey. Conventional data processing, for example common depth point (CDP) gathering, static correction, normal move-out (NMO), and the like, is performed on the time series data measured by the hydrophones in the towed array.
It has been found that vertical displacement of hydrophones in a marine seismic survey provides important benefits. One method for marine seismic prospecting using such displacement is described in U.S. Pat. No. 4,353,121. This reference discloses a towed hydrophone array where the streamer is slanted to a shallow angle from the horizontal (e.g., 1.75 degrees); the source is also towed from the same vessel in the conventional manner. As disclosed therein, the slant of the cable allows for an increasing time between the arrival of the primary and ghost reflections of the seismic wave over the length of the streamer, with the angle selected to optimally attenuate the ghost reflection in the frequency range of interest This reference discloses that during subsequent processing, including NMO, the primary signal stack from all receivers is enhanced, while the ghost signals are not enhanced since they are not time-aligned in the stack.
Another marine seismic surveying method using vertically displaced hydrophones (and sources) in a streamer is described in U.S. Pat. No. 4,254,480, where the streamer array includes hydrophones at varying depths in a V-shape, to improve the sensitivity of seismic reflections at the angles of interest and to attenuate the reception of undesired reflections.
While the towing of hydrophone arrays is a widely accepted method of gathering marine seismic data, certain limitations are inherent therein. First, the actual position of the sensors may differ from that assumed, due to currents, wind and wave action. Subsequent data processing must take such shifts in hydrophone position into account to avoid error in the result. Secondly, the time required to perform such surveys over a large area of the ocean floor may be quite significant, as many passes of the streamer vessel may be required to cover the desired area. Thirdly, the path of the vessel may be blocked from the area of interest, for example if an existing offshore drilling platform is in the survey area (as is often the case in areas of interest, particularly where additional seismic surveys are useful to supplement drilling information).
Accordingly, another method of marine seismic prospecting utilizes geophones, such as used in on-land prospecting, or alternatively hydrophones, placed on the seafloor in a single line or in a set of parallel lines Communication of the sensed information to the surface is performed by hardwired connection, or by telemetry. U.S. Pat. No. 2,590,531 and U.S. Pat. No. 3,212,600 describe examples of arrangements of seismic sensors on the seafloor After placement of the line or lines of sensors, the seismic survey is performed in the usual manner by energizing a seismic source in the water, and with the bottom-deployed sensors recording data in the conventional manner. While seismic surveys using such bottom sensors obtain the desired survey data, the cables connecting the sensors in the lines are subject to damage during deployment or retrieval, from causes such as abrasion from the seafloor during movement of the cables.
In modern surveys using bottom sensors, the position of each of the sensors may be accurately determined from the non-refracted direct arrival energy from the source. It has been found, however, that most bottom sensors in an array will receive both refracted and non-refracted energy Separation of the non-refracted energy from the total received signal, to determine the position of the sensors, requires complicated analysis techniques.
By way of further background, vertical arrays of hydrophones are useful in performing marine seismic surveys of various types. U.S. Pat. No. 4,958,328 describes a vertical array for marine vertical seismic profiling. The vertical array has a cable with hydrophones spaced therealong, and which has an anchor on one end and a floatation device at the other. Smaller buoyancy floats are mounted at the surface end of the cable, along with a telemetry buoy; the smaller floats and the buoy can move with wind and wave action, while the larger floatation device maintains the array in a vertical attitude. A vessel tows a seismic source along the profile line on either side of the vertical array to perform the 2-D or 3-D survey.
U.S. Pat. No. 4,970,697 describes another vertical seismic array, particularly directed to seismic surveys using reflected signals, particularly in locations near offshore drilling platforms. The described vertical arrays have a cable or chain which is connected to a vessel, and to which the hydrophones are mounted. One end of the cable has a weight, and the other has a float, so that the array is maintained in a vertical attitude, whether anchored to the seafloor or suspended in the water.
U.S. Pat. No. 4,694,435 describes a towable vertical seismic array, as used in refraction seismic survey operations. The array is tubular, with a weight and fins at the distal end from the vessel, and which has some internal buoyancy. The weight causes the array to be vertical when deployed, and the fins allow the array to be towed without twisting of the cable.
U.S. Pat. No. 3,539,979 describes a vertical seismic array for suspension from a buoy. The array has units suspended along the cable which, when elevated, allows hydrophones to spring horizontally away from the cable and establish the hydrophone array.
It is an object of this invention to provide an array of marine seismic sensors which are fixed in position to the seafloor, but which can be deployed and retrieved with reduced risk of damage.
It is a further object of this invention to provide such an array which provides accurate positioning information.
It is a further object of this invention to provide a system of such arrays which allows for horizontal positioning of the arrays from the surface while still maintaining a fixed bottom position.
Other objects and advantages of this invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.